The present invention relates to wireless communications, and more particularly to an efficient multiple data rate, closed-loop transmit power scheme for wireless packet communications.
The data rate of wireless data communications, including Wireless Local Area Networks (WLANs) and associated devices, continue to increase. Methods of increasing data rate includes different coding schemes and higher capacity modulation schemes. Exemplary modulation schemes include Binary Phase Shift Keying (BPSK), Quadrature PSK (QPSK), 16 Quadrature Amplitude Modulation (QAM), and 64 QAM employing corresponding constellation types with 2, 4, 16 or 64 constellation points, respectively, for modulating 1, 2, 4 or 6 bits, respectively. BPSK uses a relatively low capacity constellation type whereas 64 QAM uses a relatively high capacity constellation type. The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11a standard employs the BPSK, QPSK, 16 QAM and 64 QAM modulation schemes for WLAN operation in the 5 Gigahertz (GHz) frequency band to achieve data rates of 6, 9, 12, 18, 24, 36, 48 and 54 megabits per second (Mbps) employing orthogonal frequency division multiplexing (OFDM).
OFDM is a multi-carrier modulation technique in which data is carried on a plurality of xe2x80x9ctonesxe2x80x9d or xe2x80x9csub-carriersxe2x80x9d associated with a multi-carrier signal. In the OFDM embodiment, communication is established using packets of information including one or more synchronization data fields followed by a plurality of OFDM symbols. In an OFDM configuration, 52 sub-carrier signals are incorporated within each OFDM symbol, including 48 data tones and 4 pilot tones as known to those skilled in the art. Data is incorporated on each data tone using a selected modulation schemes, such as BPSK, QPSK, 16 QAM, and 64 QAM. Each of the modulation schemes employs a corresponding constellation type with 2, 4, 16 or 64 constellation points, respectively, for modulating 1, 2, 4 or 6 bits, respectively. The data rate range is determined by the decoding scheme and by the modulation scheme and corresponding constellation typeused. For example, BPSK is used for 6 or 9 Mbps, QPSK is used for 12 or 18 Mbps, 16 QAM is used for 24 or 36 Mbps, and 64 QAM is used for 48 or 54 Mbps. A constellation point is selected for each bit group according to the selected constellation and data rate, and each tone is modulated with an amplitude and phase according to the selected constellation point. A different data rate may be achieved for each constellation type using a different encoding technique employing different proportions of redundant information. Although the present invention is illustrated using 5 GHz OFDM, it is understood that different RF bands and modulation schemes may be used.
Increases in required transmit power is a necessary consequence of increased path loss in the 5 GHz band. With increased path loss, coverage (communications distance) is at a premium. In order to maximize distance, it is desired to push the transmit power amplifier to a maximum power limit for optimal performance. A given wireless network may be operated at multiple data rates. An Access Point (AP), for example, may communicate with several devices in a wireless area, where each device operates at a different data rate. In fact, an IEEE 802.11a OFDM-based WLAN requires transmission at multiple data rates. Maximum transmit power, however, is a function of the particular data rate of transmission, which is further related to the encoding scheme and constellation type. As the data rate increases, distortion tolerance decreases. For example, the simpler constellations (e.g. BPSK) used in the lower data rates may be transmitted with greater power, albeit higher distortion. As the constellation type becomes more complex, the distance between constellation points decreases (and the corresponding transmit waveform becomes more complex). Distortion tolerance is consequently reduced. Also, a higher data rate for a given constellation type using less redundancy information is more sensitive to distortion and therefore less robust. Because lower data rates can sustain more distortion, they can be transmitted with greater total power.
In this manner, to increase output power (range), it is desired to transmit with as much distortion as possible. On the other hand, in order to prevent serious distortions and spectral leakage into adjacent channels, the power amplifier output should be reduced to limit the effect of waveform peak saturation. As a result of these competing considerations, the target power level is different for each data rate and/or constellation type employed. The problem arises as to how to efficiently maintain closed-loop power control in a radio (such as an AP or the like) that transmits multiple data rate packets at multiple output power levels. Two primary means of automatic level control (ALC) control are available, including an open loop method and a closed loop method. In the open loop method, a control device, such as a media access control (MAC) device or the like, simply writes a default controlling set point value to a register. The controlling set point value may be determined at time of manufacture using a calibration procedure. The use of default values carry the penalty of being less than optimal since conservative assumptions must be made. Differences from channel to channel are less likely to be a problem and can be minimized by interpolation. A more severe concern is that the radio may drift over time causing significant distortion and errors. Without closed loop control, unconstrained power drifts may cause spectral violations and bleed-over into adjacent channels thereby interfering with other users.
In the closed loop method, the control device initially sets a nominal set point value and closes the loop by sampling the output transmit power and driving it to the desired set point. The power amplifier, however, is a non-linear device. The efficiency, distortion, nonlinearity and interference issues dictate a different set point for each data rate or modulation scheme used. In this manner, a single control loop is insufficient to achieve the optimal power set point for each constellation type or data rate and separate loops would otherwise be required. Separate control loops contribute to the complexity and cost of radio devices. Furthermore, certain data rates may only be used infrequently so that significant drift may lead to significant error over time. In other words, infrequently used loops exhibit significant errors and inefficient operation.
A power controller according to an embodiment of the present invention is used for a wireless transmitter that operates using multiple constellation types to achieve several different data rates, where the transmitter includes a variable output power amplifier having a gain input for controlling output power. The power controller includes an output power detector, an error circuit and a power control circuit. The output power detector provides an output power level value indicative of output power. The error circuit compares the output power level value with a target power value and asserts an error value indicative thereof. The power control circuit asserts a power control signal based on a power control value for controlling the gain input of the output power amplifier, where the power control circuit regulates the power control value based on the error value. In order to maximize or otherwise optimize operation for each of the multiple data rates, the power controller further includes an input normalization circuit that selectively offsets the output power level value using one or more input scale values selected by a data rate select signal. Also, the power controller includes an output normalization circuit that selectively offsets the power control value using one or more output scale values selected by the data rate select signal.
It is appreciated that typical power amplifier used for wireless communications is a nonlinear device. Also, the maximum power level achievable for each data rate is different since based on either maximal spectral leakage or distortion metrics. Nonetheless, it is desired to maximize output power for each data rate to maximize range and operating efficiency. Lower data rates using less complex constellation types and/or encoding techniques are more tolerant of distortion, so that the radio may be operated at higher power levels with increased distortion although within spectral leakage interference metrics. Higher data rates using more complex constellation types and/or encoding schemes are less tolerant to distortion and more likely to be power-limited based on maximum allowable distortion levels. A power controller as described herein employs a single loop with separate input and output correction factors for each data rate or constellation type to maximize output power for all data rates using the same output power amplifier. Correction applied in the loop for any one data rate applies to all other data rates so that all packets contribute to loop adaptation.
The target power level may be based on a first data rate or constellation type. If so, the corresponding input and output scale values are both zero. Such configuration provides greater efficiency since correction factors are not necessary for the first or nominal data rate or constellation type. Alternatively, the target power level is arbitrarily determined and correction factors are used for each constellation type.
The input and output normalization circuits may offset the output power level value or the power control value, respectively, based on a selected one of a plurality of input or output scale values, respectively. Each scale value corresponds to one of the plurality of data rates. In one embodiment, the data rates are achieved using multiple modulation schemes, including BPSK, QPSK, QAM 16 and QAM 64. The input normalization circuit may include a first memory that stores a plurality of input scale values and a first summing junction that combines the output power level value with one of the plurality of input scale values as selected by the data rate select signal. Also, the output normalization circuit may include a second memory that stores a plurality of output scale values and a second summing junction that combines the power control value with a selected one of the plurality of output scale values as selected by the data rate select signal.
In more specific embodiments, the output power detector may include an average circuit that measures and averages a plurality of output power level samples to generate an averaged output power level value. The power control circuit may further include a single loop accumulator that adjusts the power control value by the error value for each data rate or constellation type. Also, the input and output scale values may correspond to constellation types rather than specific data rates. A loop regulator may be provided to scale the error value by a predetermined amount to control loop convergence timing.
The present disclosure describes a radio frequency (RF) communication device that includes a power control loop for controlling output power for each data rate or constellation type. The RF communication device includes an output portion, a Baseband processor and a control device, where the control device further includes input and output normalization adjust circuits. The output portion includes a power detector and a variable output power amplifier having a gain input. The Baseband processor receives a power level signal from the power detector and provides a corresponding power level value. The Baseband processor also receives a power control value and asserts a power control signal to the gain input of the variable output power amplifier. The Baseband processor may include an averaging circuit that samples the power level signal a plurality of times and that averages the samples.
The control device receives and compares an adjusted power level value with a predetermined set point value and generates a corresponding error value and adjusts the power control value based on the error value. The control device uses a data rate signal indicative of a selected data rate or constellation type. The input normalization adjust circuit includes a first memory that stores one or more input adjustment values selected by the data select signal for adjusting the power level value. The output normalization adjust circuit includes a second memory that stores one or more output adjustment values selected by the data select signal for adjusting the power control value.
The set point value may be based on a first (or nominal) data rate or constellation type so that a corresponding pair of input and output adjustment values are zero. The first and second memories may store a plurality of input and output adjustment values, where each pair of input and output adjustment values is employed to maximize output power when operating at corresponding data rates. The first and second memories may each comprises a plurality of programmable registers.
The control device may be implemented in any appropriate manner to achieve the desired results. For example, the control device may include a first summing junction that adds a power level value with a selected input adjustment value to provide an adjusted power level value, and a second summing junction that compares the adjusted power level value with the set point value to generate the error value. The control device may include a loop regulator that scales the error value to control convergence timing of the power control loop. The control device may include an accumulator that stores the power control value and a third summing junction that adjusts the power control value in the accumulator by the scaled error value. The control device may further include a fourth summing junction that adds the adjusted power control value from the accumulator and a selected output adjustment value and that provides an adjusted power control value. The control device may include a third memory that stores an initial power control value that is copied into the accumulator upon power up or reset. The initial power control value may be based on a percentage of a measured power control value that achieves a maximum power level for a nominal constellation type. The control device may be a media access control (MAC) device.
The present disclosure describes a method of calibrating a radio frequency (RF) transmitter to maximize output power for each of a plurality of data rates or constellation types. The RF transmitter may use different constellation types and/or encoding schemes to achieve multiple data rates. In any case, the RF transmitter includes a single power control loop that converges using a target power set point in a first memory during normal operation that is used for all data rates or constellation types. The RF transmitter further includes a second memory for storing a plurality of output scale factors used to adjust a power control value and a third memory for storing a plurality of input scale factors used to adjust a power level value. The input and output scale factors are selected by a data rate signal indicative of data rate or constellation type.
The method includes opening the control loop during calibration, operating the RF transmitter using a selected first data rate or constellation type while measuring output power metrics including a spectral leakage metric and a distortion metric, applying and adjusting a power control value at an input of the opened control loop until a maximum desired power level is achieved for the data rate or constellation type when either one of the spectral leakage metric and the distortion metric is maximized (resulting in a first power control value), storing a power level value at an output of the opened control loop into the first memory as the target power set point, and operating the RF transmitter with the opened control loop for each of one or more additional data rates or constellation types while measuring the spectral leakage and distortion metrics. For each additional data rate or constellation type, the method includes determining a new power control value and a new power level value when a new maximum desired power level is achieved for a corresponding data rate or constellation type when either one of the spectral leakage metric and the distortion metric is maximized, storing an output scale factor for the corresponding data rate or constellation type in the second memory, where the output scale factor represents a differential between the first power control value and the new power control value, and storing an input scale factor for the corresponding data rate or constellation type in the third memory, where the input scale factor represents a differential between the target power set point and the new power level value.
The RF transmitter may include a fourth memory for storing an initial power control value and an accumulator used to converge the control loop during closed loop operation. The initial power control value is copied to the accumulator during power up or reset. In this case, the method may further include storing a predetermined percentage of the first power control value into the fourth memory as the initial power control value.